findprog-in: Allow overriding the current directory. * lib/findprog.h (find_in_given_path): Add directory argument. * lib/findprog-in.c (find_in_given_path): Likewise. * lib/execute.c (execute): Update caller. * lib/spawn-pipe.c (create_pipe): Likewise. * lib/windows-spawn.c (spawnpvech): Likewise. * NEWS: Mention the change.